fix: detect context length for custom model endpoints via fuzzy matching + config override (#2051)

* fix: detect context length for custom model endpoints via fuzzy matching + config override

Custom model endpoints (non-OpenRouter, non-known-provider) were silently
falling back to 2M tokens when the model name didn't exactly match what the
endpoint's /v1/models reported. This happened because:

1. Endpoint metadata lookup used exact match only — model name mismatches
   (e.g. 'qwen3.5:9b' vs 'Qwen3.5-9B-Q4_K_M.gguf') caused a miss
2. Single-model servers (common for local inference) required exact name
   match even though only one model was loaded
3. No user escape hatch to manually set context length

Changes:
- Add fuzzy matching for endpoint model metadata: single-model servers
  use the only available model regardless of name; multi-model servers
  try substring matching in both directions
- Add model.context_length config override (highest priority) so users
  can explicitly set their model's context length in config.yaml
- Log an informative message when falling back to 2M probe, telling
  users about the config override option
- Thread config_context_length through ContextCompressor and AIAgent init

Tests: 6 new tests covering fuzzy match, single-model fallback, config
override (including zero/None edge cases).

* fix: auto-detect local model name and context length for local servers

Cherry-picked from PR #2043 by sudoingX.

- Auto-detect model name from local server's /v1/models when only one
  model is loaded (no manual model name config needed)
- Add n_ctx_train and n_ctx to context length detection keys for llama.cpp
- Query llama.cpp /props endpoint for actual allocated context (not just
  training context from GGUF metadata)
- Strip .gguf suffix from display in banner and status bar
- _auto_detect_local_model() in runtime_provider.py for CLI init

Co-authored-by: sudo <sudoingx@users.noreply.github.com>

* fix: revert accidental summary_target_tokens change + add docs for context_length config

- Revert summary_target_tokens from 2500 back to 500 (accidental change
  during patching)
- Add 'Context Length Detection' section to Custom & Self-Hosted docs
  explaining model.context_length config override

---------

Co-authored-by: Test <test@test.com>
Co-authored-by: sudo <sudoingx@users.noreply.github.com>

agent/context_compressor.py

@@ -46,6 +46,7 @@ class ContextCompressor:
         summary_model_override: str = None,
         base_url: str = "",
         api_key: str = "",
+        config_context_length: int | None = None,
     ):
         self.model = model
         self.base_url = base_url
@@ -56,7 +57,10 @@ class ContextCompressor:
         self.summary_target_tokens = summary_target_tokens
         self.quiet_mode = quiet_mode
 
-        self.context_length = get_model_context_length(model, base_url=base_url, api_key=api_key)
+        self.context_length = get_model_context_length(
+            model, base_url=base_url, api_key=api_key,
+            config_context_length=config_context_length,
+        )
         self.threshold_tokens = int(self.context_length * threshold_percent)
         self.compression_count = 0
         self._context_probed = False  # True after a step-down from context error

agent/model_metadata.py

@@ -136,6 +136,8 @@ _CONTEXT_LENGTH_KEYS = (
     "max_input_tokens",
     "max_sequence_length",
     "max_seq_len",
+    "n_ctx_train",
+    "n_ctx",
 )
 
 _MAX_COMPLETION_KEYS = (
@@ -342,6 +344,25 @@ def fetch_endpoint_model_metadata(
                     entry["pricing"] = pricing
                 _add_model_aliases(cache, model_id, entry)
 
+            # If this is a llama.cpp server, query /props for actual allocated context
+            is_llamacpp = any(
+                m.get("owned_by") == "llamacpp"
+                for m in payload.get("data", []) if isinstance(m, dict)
+            )
+            if is_llamacpp:
+                try:
+                    props_url = candidate.rstrip("/").replace("/v1", "") + "/props"
+                    props_resp = requests.get(props_url, headers=headers, timeout=5)
+                    if props_resp.ok:
+                        props = props_resp.json()
+                        gen_settings = props.get("default_generation_settings", {})
+                        n_ctx = gen_settings.get("n_ctx")
+                        model_alias = props.get("model_alias", "")
+                        if n_ctx and model_alias and model_alias in cache:
+                            cache[model_alias]["context_length"] = n_ctx
+                except Exception:
+                    pass
+
             _endpoint_model_metadata_cache[normalized] = cache
             _endpoint_model_metadata_cache_time[normalized] = time.time()
             return cache
@@ -439,16 +460,26 @@ def parse_context_limit_from_error(error_msg: str) -> Optional[int]:
     return None
 
 
-def get_model_context_length(model: str, base_url: str = "", api_key: str = "") -> int:
+def get_model_context_length(
+    model: str,
+    base_url: str = "",
+    api_key: str = "",
+    config_context_length: int | None = None,
+) -> int:
     """Get the context length for a model.
 
     Resolution order:
+    0. Explicit config override (model.context_length in config.yaml)
     1. Persistent cache (previously discovered via probing)
     2. Active endpoint metadata (/models for explicit custom endpoints)
     3. OpenRouter API metadata
     4. Hardcoded DEFAULT_CONTEXT_LENGTHS (fuzzy match for hosted routes only)
     5. First probe tier (2M) — will be narrowed on first context error
     """
+    # 0. Explicit config override — user knows best
+    if config_context_length is not None and isinstance(config_context_length, int) and config_context_length > 0:
+        return config_context_length
+
     # 1. Check persistent cache (model+provider)
     if base_url:
         cached = get_cached_context_length(model, base_url)
@@ -458,13 +489,30 @@ def get_model_context_length(model: str, base_url: str = "", api_key: str = "")
     # 2. Active endpoint metadata for explicit custom routes
     if _is_custom_endpoint(base_url):
         endpoint_metadata = fetch_endpoint_model_metadata(base_url, api_key=api_key)
-        if model in endpoint_metadata:
-            context_length = endpoint_metadata[model].get("context_length")
+        matched = endpoint_metadata.get(model)
+        if not matched:
+            # Single-model servers: if only one model is loaded, use it
+            if len(endpoint_metadata) == 1:
+                matched = next(iter(endpoint_metadata.values()))
+            else:
+                # Fuzzy match: substring in either direction
+                for key, entry in endpoint_metadata.items():
+                    if model in key or key in model:
+                        matched = entry
+                        break
+        if matched:
+            context_length = matched.get("context_length")
             if isinstance(context_length, int):
                 return context_length
         if not _is_known_provider_base_url(base_url):
             # Explicit third-party endpoints should not borrow fuzzy global
             # defaults from unrelated providers with similarly named models.
+            logger.info(
+                "Could not detect context length for model %r at %s — "
+                "defaulting to %s tokens (probe-down). Set model.context_length "
+                "in config.yaml to override.",
+                model, base_url, f"{CONTEXT_PROBE_TIERS[0]:,}",
+            )
             return CONTEXT_PROBE_TIERS[0]
 
     # 3. OpenRouter API metadata